The majority of polyethylene insulated low voltage cables are cured or crosslinked by a moisture cure process whereby an alkoxy silane attached to the polyethylene chain is hydrolyzed and then cures under the influence of a suitable catalyst. The alkoxy silane may be attached to the polyethylene chain by two methods. Either vinyl trialkoxy silane is copolymerized with ethylene to yield a silane copolymer or the vinyl alkoxy silane is grafted onto the polyethylene, polymer backbone by peroxide initiated reactive extrusion.
In the former case the resulting copolymer is similar to LDPE and is a semi-rigid insulation material (flexural modulus of about 200 MPa). In the second case, more flexible resin systems can be produced by blending in an elastomer with the polyethylene during a reactive extrusion step. However, special extrusion equipment (e.g., silane dosing equipment, designed barrier screw, etc.) is needed to carry out the radical grafting process successfully. Additionally, elastomers that degrade or scission in the presence of peroxide or other free-radical sources cannot be used. Moreover, existing moisture-cured resins tend to become brittle or stiff at low temperatures, such as can be encountered in outdoor use during the winter.
Thus there is still a need for a simple process to prepare silane crosslinkable polyolefins with improved flexibility which still meet tensile, elongation and cure state target properties for low voltage insulation, particularly at low temperatures.